

.collapse {
    font-size: 14px;
    width: 35px;
    height: 35px;
    background-size: 12px 7px;
    background-position: center center;
    background-repeat: no-repeat;
}

.arrow-down {
    background-image: url("");
}

.arrow-up {
    transform: rotate(180deg);
    background-image: url("");
}